In the hypothalamus, neurons that synthesize melanin concentrating hormone (MCH) play a key role in the initiation of feeding and in energy homeostasis. MCH antagonists not only reduce obesity, they are also anxiolytic and modulate the sleep-wake state. MCH application in the brain increases feeding and weight gain, whereas loss of the MCH gene causes a decrease. Despite a large amount of fascinating data showing many effects of MCH and MCH neurons, we understand remarkably little about how the neurons act in the brain, and how MCH functions at the cellular level. The primary focus here is to study the MCH neurons, and the actions of MCH at the neuronal level. Whole cell patch clamp recording will be done in transgenic mouse hypothalamic slices containing MCH neurons identifiable by selective expression of GFP. It is surprising that relatively little postsynaptic actions of MCH have been detected. We will test three brain regions that get a high level of MCH axon innervations, the highest level of receptors, and contain a high density of MCH- GFP axons that will be used to target postsynaptic neurons to test MCH responses. These three areas, the hypothalamic dorsomedial nucleus, the medial mammillary nucleus, and locus coeruleus all play key roles in the functions attributed to MCH neurons. A second hypothesis we will test is that heightened activity unmasks MCH actions on postsynaptic targets or presynaptic sites. A third hypothesis tests the view that MCH neurons are a homogeneous group of cells, and suggests that there are two populations of MCH cells, small and large, with different dendritic arbors, axonal projections, and physiological characteristics. We propose to study differences in the dendritic arbors and axonal projections of these cells with a viral vector, dG-vesicular stomatitis virus and pseudorabies virus that express red fluorescent reporters. Ultrastructural differences in the two cell types will be examined with electron microscopy. Differences in active and passive membrane properties and responses to neuromodulators will be studied with whole cell recording in mouse brain slices. In preliminary work, we find that application of the inhibitory amino acid transmitter GABA can result in an unexpected but substantial depolarization of adult MCH neurons. We will test the hypothesis that these GABA actions are not based on Cl- flux, and that they can lead to an increase in spikes in certain physiological states. A current model of energy homeostasis suggests that the orexigenic NPY cells from the actuate nucleus excite MCH neurons. But we find NPY is inhibitory to MCH cells. A more tractable model that we propose to test includes an inhibitory interneuron in the lateral hypothalamus: actuate nucleus NPY inhibits the GABA interneuron, leading to disinhibition of the downstream MCH neuron. We will use a combination of whole cell recording, tract tracing, and immunocytochemistry to test this model. Obesity has become a major health problem leading to increases in heart disease, strokes, mortality and other risks. Studies of the cellular physiology of MCH neurons should provide a better understanding of the role of this system in regulating energy homeostasis, obesity, and anxiety, and will help us understand how we might regulate the MCH system to ameliorate human disease.